427 research outputs found
Jets in neutron star X-ray binaries: a comparison with black holes
(Abridged) We present a comprehensive study of the relation between radio and
X-ray emission in neutron star X-ray binaries, use this to infer the general
properties of the disc-jet coupling in such systems, and compare the results
quantitatively with those already established for black hole systems. There are
clear qualitative similarities between the two classes of object: hard states
below about 1% of the Eddington luminosity produce steady jets, while transient
jets are associated with outbursting and variable sources at the highest
luminosities. However, there are important quantitative differences: the
neutron stars are less radio-loud for a given X-ray luminosity (regardless of
mass corrections), and they do not appear to show the strong suppression of
radio emission in steady soft states which we observe in black hole systems.
Furthermore, in the hard states the correlation between radio and X-ray
luminosities of the neutron star systems is steeper than the relation observed
in black holes by about a factor of two. This result strongly suggests that the
X-ray emission in the black hole systems is radiatively inefficient, with an
approximate relation of the form L_X \propto \dot{m}^2, consistent with both
advection-dominated models and jet-dominated scenario. On the contrary the jet
power in both classes of object scales linearly with accretion rate. This
constitutes some of the first observational evidence for the radiatively
inefficient scaling of X-ray luminosity with accretion rate in accreting black
hole systems.Comment: Accepted for publication in MNRA
Linking Jet Emission, X-ray States and Hard X-ray Tails in the Neutron Star X-ray Binary GX 17+2
We present the results from simultaneous radio (Very Large Array) and X-ray
(Rossi-X-ray Timing Explorer) observations of the Z-type neutron star X-ray
binary GX~17+2. The aim is to assess the coupling between X-ray and radio
properties throughout its three rapidly variable X-ray states and during the
time-resolved transitions. These observations allow us, for the first time, to
investigate quantitatively the possible relations between the radio emission
and the presence of the hard X-ray tails and the X-ray state of the source. The
observations show: 1) a coupling between the radio jet emission and the X-ray
state of the source, i.e. the position in the X-ray hardness-intensity diagram
(HID); 2) a coupling between the presence of a hard X-ray tail and the position
in the HID, qualitatively similar to that found for the radio emission; 3) an
indication for a quantitative positive correlation between the radio flux
density and the X-ray flux in the hard-tail power law component; 4) evidence
for the formation of a radio jet associated with the Flaring Branch-to-Normal
Branch X-ray state transition; 5) that the radio flux density of the
newly-formed jet stabilizes when also the normal-branch oscillation (NBO) in
the X-ray power spectrum stabilizes its characteristic frequency, suggesting a
possible relation between X-ray variability associated to the NBO and the jet
formation. We discuss our results in the context of jet models.Comment: Accepted by Ap
Evidence of a decrease of kHz quasi-periodic oscillation peak separation towards low frequencies in 4U 1728-34 (GX 354-0)
We have produced the colour-colour diagram of all the observations of 4U 1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996 to 2002) and found observations filling in a previously reported `gap' between the island and the banana X-ray states. We have made timing analysis of these gap observations and found, in one observation, two simultaneous kHz quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit in the overall trend of the source. The `lower' kHz QPO has a centroid frequency of ~308 Hz. This is the lowest `lower' kHz QPO frequency ever observed in 4U 1728-34. The peak frequency separation between the `upper' and the `lower' kHz QPO is ĪĪ½= 274 +/- 11 Hz, significantly smaller than the constant value of ĪĪ½~ 350 Hz found when the `lower' kHz QPO frequency is between ~500 and 800 Hz. This is the first indication in this source for a significant decrease of kHz QPO peak separation towards low frequencies. We compare the result briefly to theoretical models for kHz QPO production
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
Correlation between radio luminosity and X-ray timing frequencies in neutron star and black hole X-ray binaries
We report on correlations between radio luminosity and X-ray timing features
in X-ray binary systems containing low magnetic field neutron stars and black
holes. The sample of neutron star systems consists of 4U 1728-34, 4U 1820-34,
Ser X-1, MXB 1730-335, GX 13+1, the millisecond X-ray pulsars SAX J1808.4-3658
and IGR J00291+5934, and these are compared with the black hole system GX
339-4. The analysis has been done using data from pointed observations of the
Rossi X-ray Timing Explorer coordinated with radio observations. In the neutron
star systems the radio luminosity L_{R} is correlated with the characteristic
frequency of the L_{h} Lorentzian component detected contemporaneously in the
power spectrum, and anticorrelated with its strength. Similarly, in the black
hole system GX 339-4 L_{R} is correlated with the frequency of the L_{\ell}
component in the power spectrum, and anti-correlated with its strength. The
index of a power-law fit to the correlation is similar in both cases, L_{R}
\propto \nu^{~1.4} and L_{R} \propto (rms)^{-2.3}. At lower timing frequencies,
the radio luminosity is further found to be correlated with the characteristic
(break) frequency of the L_{b} component of the power spectra in the neutron
stars and, marginally, with the equivalent break frequency in GX 339-4. We
briefly discuss the coupling between the innermost regions of the accretion
disc and the production of the jet and, from the behaviour of the ms accreting
X-ray pulsars, the possible role of the NS magnetic field.Comment: Accepted for publication in MNRA
Millihertz Quasi-periodic Optical Oscillations in 4U 0614+091
We report the discovery of a 1mHz optical quasi-periodic oscillation (QPO) in
the candidate ultracompact low-mass X-ray binary 4U 0614+091. The ultra-low
frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is
likely a signature of structure in the accretion disk. The QPO can be
reasonably fitted with a single sine wave but with a phase jump part way
through the observation, indicating that it is not coherent.We also identify a
48 min modulation, approximately consistent with the suggested orbital period
of O'Brien (2005) and Shahbaz et al. (2008). If this is indeed orbital, it
supports an identification of 4U 0614+091 as an ultra-compact source.Comment: 7 pages, 5 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322
We report results from a systematic study of the spectral energy distribution
(SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The
jets of both sources have been directly imaged at both radio and X-ray
frequencies, which makes it possible to constrain the spectrum of the radiating
electrons in the jets. We modelled the observed SEDs of the jet `blobs' with
synchrotron emission alone and with synchrotron emission plus inverse Compton
scattering. The results favor a pure synchrotron origin of the observed jet
emission. Moreover, we found evidence that the shape of the electron spectral
distribution is similar for all jet `blobs' seen. Assuming that this is the
case for the jet as a whole, we then applied the synchrotron model to the radio
spectrum of the total emission and extrapolated the results to higher
frequencies. In spite of significant degeneracy in the fits, it seems clear
that, while the synchrotron radiation from the jets can account for nearly 100%
of the measured radio fluxes, it contributes little to the observed X-ray
emission, when the source is relatively bright. In this case, the X-ray
emission is most likely dominated by emission from the accretion flows. When
the source becomes fainter, however, the jet emission becomes more important,
even dominant, at X-ray energies. We also examined the spectral properties of
the sources during outbursts and the correlation between the observed radio and
X-ray variabilities. The implication of the results is discussed.Comment: 9 pages, 11 figures, MNRAS, accepted; the paper has been much
expanded (e.g., arguments strengthened, another source H 1743-322 added) and
rewritten (e.g., title changed, abstract revised); the main conclusions
remain unchange
Coupled radio and X-ray emission and evidence for discrete ejecta in the jets of SS 433
We present five epochs of simultaneous radio (VLA) and X-ray (Chandra)
observations of SS 433, to study the relation between the radio and X-ray
emission in the arcsecond-scale jets of the source. We detected X-ray emission
from the extended jets in only one of the five epochs of observation,
indicating that the X-ray reheating mechanism is transient. The reheating does
not correlate with the total flux in the core or in the extended radio jets.
However, the radio emission in the X-ray reheating regions is enhanced when
X-ray emission is present. Deep images of the jets in linear polarization show
that outside of the core, the magnetic field in the jets is aligned parallel to
the local velocity vector, strengthening the case for the jets to be composed
of discrete bullets rather than being continuous flux tubes. We also observed
anomalous regions of polarized emission well away from the kinematic trace,
confirming the large-scale anisotropy of the magnetic field in the ambient
medium surrounding the jets.Comment: Accepted for publication in ApJ. 11 pages (emulateapj), 9 figures, 3
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